The present invention is particularly adapted to the prevention of chip build-up in the central coolant tank of a large central coolant system for the machining of light metals, such as aluminum, magnesium, titanium and the like. The machining chips from objects being machined of such metals are generally large, are of curled or convolute configuration, and have a large surface area relative to their weight. Further, the presence of any tramp oil in the system will inhibit adequate wetting of the chips by the coolant.
As a result, the chips merely accumulate on the surface of the pool of coolant maintained in the filtration tank, and a layer of such chips may build up to a substantial depth on the surface of the liquid in the tank. The presence of any tramp oil in the coolant, particularly at the pool surface, inhibits or prevents wetting of the chips with the coolant and increases the chip build-up. The presence of such a buoyant layer of chips interferes materially with the normal filtration operation, since the floating chips can become entangled in a drag-out conveyor for the tank, thus jamming the conveyor, and the chip layer can hang up on any overflow weirs or other portions of the filtration mechanism located at or near the surface of the tank. If a skimmer conveyor is provided for the surface of the tank, the chip build-up also will interfere with the operation of the skimmer conveyor, again causing jamming and interferece with the normal filtration operation.
The present invention proposes a simple solution to the problem of chip build-up, namely the injection of a gas, preferably air, into the body of liquid in the filtration tank so that the air bubbling up through the liquid will agitate the liquid beneath the chip layer and through the depth of the chip layer. As the bubbles rise through the liquid, the hydrostatic pressure on each bubble decreases progressively, so that the bubbles become larger as they travel upwardly through the liquid. The bubbles then burst when exposed to the liquid-air interface, and the coolant from the burst bubbles and other coolant agitated by the bubbles will wet the chips. The chips, once adequately wet with the coolant, will sink through the body of coolant for removal during normal chip-coolant separation, both by the conventional drag-out conveyor and at the filtration apparatus immersed in the tank.
Surprisingly, it has been found that the upward bubbling of the air through the coolant does not cause any substantial frothing of the coolant, nor does it cause any undue loss of the coolant due to evaporation. Further, it has been found that the air bubbling technique of the present invention will adequately wet the chips so that they sink despite the presence of tramp oil in the body of coolant. The present invention prevents or minimizes the effect of such tramp oil.
In one embodiment of the present invention, the air is simply introduced into the tank through a series of pipes located adjacent the bottom of the tank, with the air bubbling upwardly through the coolant. In another embodiment of the present invention, the air conduits are integrated into the filtration mechanism, so that no extraneous structure is necessary to perform the bubbling function. Further, such integral air conduits are distributed throughout the tank at the locations where the chip-coolant separation is accomplished.
The air may be continuously introduced in relatively small volumes or, preferably, the air is introduced periodically on a predetermined schedule related to the filtration function. For example, where the filtration mechanism is a cylindrical drum which is rotated against a scraping mechanism for the removal of accreted contaminants from the surface of the drum, the air is introduced each time that the drum is indexed and as a consequence of drum indexing. Alternatively, a separate timing device may be used to periodically inject the air. It has been found that injecting the air periodically by either means conserves air while providing a sufficient flow of coolant-agitating bubbles, and the non-wetted layer of chips on the surface of the coolant in the tank can either be minimized or eliminated completely by such periodic injection.
Thus, it will be seen that the present invention provides an extremely simple, effective and inexpensive method for preventing the build-up of chips on the surface of the coolant in the filtration tank.